The present invention is directed to a process for manufacture of fine precious metal-based particles, specifically to a process for manufacture of silver-based nanoparticles and electrical contact materials via an intermediate silver (+1)-oxide species. Furthermore, methods for use of the silver-based particles in conductive inks and in antimicrobial applications are disclosed.
In recent years, fine metallic particles, particularly nanoparticles, of definite shape and size have received considerable interest and attention because of their fascinating properties and potential applications, e.g. in semiconductors, consumer products, opto-electronics, electronics, catalysis, transportation, energy, medical sciences and biotechnology. The intrinsic properties of fine metallic particles are mainly determined by their size, shape, composition, crystallinity and structure.
A number of techniques have been proposed for the preparation of fine precious metal particles, including alcohol reduction, the polyol process, sonochemical methods, decomposition of organometallic precursors, vaporisation-condensation methods and electrolysis of bulk metals.
Generally, metallic silver particles are prepared in a reduction process employing reducing agents such organic acids, alcohols, polyols, aldehydes, sugars etc. (ref to D. V. Goia, E. Matijevic, New. J. Chem. 1998, pages 1203-1215).
In this direct (i.e. 1-step) process, a suitable Ag(+1) compound (which contains silver in the oxidation state +1) is reduced in an acidic environment (i.e. pH 0 to 5) to metallic silver with oxidation state 0.
The chemical reducing agents commonly used are toxic and/or carcinogenic compounds (e.g. hydrazine, sodium borohydride, formaldehyde) and cause safety and health problems in volume production.
In the well known polyol process, silver nanoparticles are prepared by the reduction of silver nitrate with ethylene glycol at about 160° C. The ethylene glycol serves as both reductant and solvent. Typically, stabilizing agents such as polyvinylpyrolidone (PVP) are employed (ref to Y. Sun and Y. Xia, Science, Vol. 298, 2176-2179 (2002)).
The drawbacks with this process are the high energy consumption, the use of expensive organic glycol solvent and the recycling of waste solvent after use.
In addition to the methods described above, the preparation of silver particles via the intermediates silver hydroxide (AgOH), silver carbonate (Ag2CO3) or silver oxide (Ag2O) is state of the art. These 2-step processes require the addition of a base, usually an alkali hydroxide such as NaOH or KOH, to form an intermediate Ag(+1)-oxide species. This intermediate species is subsequently reduced by the addition of a reducing agent.
JP 60 077 907 discloses the manufacture of silver dust, wherein an aqueous silver nitrate solution is neutralized with an aqueous alkali hydroxide to form a slurry containing a silver oxide precipitate. The slurry is reduced by adding a reducing agent to prepare the silver particles.
DE 1 185 821 teaches a 2-step process for manufacture of silver powder via reduction of precipitated silver oxide with formaldehyde.
The intermediate Ag(+1)-oxide species can also be reduced by a thermal treatment. EP 370 897B1 discloses a 2-step process for manufacture of silver/tin oxide contact materials, by precipitation of Ag2O with a strong base in the presence of tin oxide. In a further step, the silver oxide is thermally reduced at temperatures in the range of 200 to 500° C. to form metallic silver.
The high temperatures employed cause high energy costs due to heating of the reaction mixtures. Furthermore, due to the boiling point of water, the required temperatures cannot be achieved in water-based reaction processes. Therefore additional separation steps are necessary prior to the heat treatment. As a result, the production process according to EP 370 897B1 is expensive and complex.
In summary, the presently known processes for preparation of silver particles (either by 1-step or by 2-step processes) are not sufficient in terms of environmental safety, process simplicity, raw materials costs and energy costs.
It was an objective of the present invention to provide a new process for manufacture of precious metal-based particles, particularly silver-based particles.
It was another objective of the present invention to provide a new process for manufacture of silver-based electrical contact materials.
These processes should be, for example, versatile, simple, straight-forward, energy-saving, cost-efficient and environmentally friendly.
It was a further objective of the present invention to provide silver nanoparticles and silver-based electrical contact materials with improved material characteristics.
These objectives are met by the processes and products of the present invention.